Power oscillator apparatus with transformer-based power combining

ABSTRACT

An oscillator circuit includes first and second oscillators arranged in a series configuration between a supply voltage node and a reference voltage node. The first and second oscillators are configured to receive a synchronizing signal for controlling synchronization in frequency and phase. An electromagnetic network provided to couple the first and the second oscillators includes a transformer with a primary circuit and a secondary circuit. The primary circuit includes a first portion coupled to the first oscillator and second portion coupled to the second oscillator. The first and second portions are connected by a circuit element for reuse of current between the first and second oscillators. The oscillator circuit is fabricated as an integrated circuit device wherein the electromagnetic network is formed in metallization layers of the device. The secondary circuit generates an output power combining power provided from the first and second portions of the primary circuit.

PRIORITY CLAIM

This application claims priority from Italian Application for Patent No.MI2013A000454 filed Mar. 26, 2013, the disclosure of which isincorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a power oscillator apparatus withtransformer-based power combining

BACKGROUND

It is known in the state of the art the use of circuit apparatuscomprising at least two oscillators coupled by means of a propernetwork. The main applications of such an apparatus are theimplementation of both quadrature signals and voltage-controlledoscillators with low phase-noise. For this approach, the design of thecoupling network is the main issue. The coupling network may be of theactive type, as disclosed in Jeong Ki Kim et al., “A current-reusequadrature VCO for wireless body area networks,” IEEE/NIH LiSSA, pp.55-58, 2011 (the disclosure of which is incorporated by reference), orcapacitive type as disclosed in Oliveira, L. B. et al., “Synchronizationof two LC-oscillators using capacitive coupling,” IEEE ISCAS, pp.2322-2325, 2008 (the disclosure of which is incorporated by reference),or inductive type as disclosed in Tzuen-Hsi Huang et al., “A 1 V 2.2 mW7 GHz CMOS quadrature VCO using current-reuse and cross-coupledtransformer-feedback technology,” IEEE MWCL, vol. 18, pp. 698-700,October 2008 (the disclosure of which is incorporated by reference).

Also it is known in the state of the art the use of power combiningtechniques to increase the overall output power in several applications.Due to technology limits, (e.g., breakdown, electro-migrationconstraints, thermal issues, etc.) the power level delivered by a singlepower stage is often below the application requirements, thus amultistage solution is required. When it comes about dc/ac conversion,transformer-based power-combining is the straight-forward solution. Anexample of power-combining system is disclosed in Tomita et al., “1-W3.3-16.3-V boosting wireless power oscillator circuits with vectorsumming power controller,” IEEE JSSC, vol. 47, pp. 2576-2585, November2012 (the disclosure of which is incorporated by reference), where twopower stages separately drive two series resonant circuits and bothdrivers are magnetically coupled with the secondary inductance. Bycontrolling the phase relation between the driver's signals, the outputpower can effectively reach two times the power delivered by a singlestage.

SUMMARY

One aspect of the present disclosure is to provide a power oscillatorapparatus with transformer-based power combining which is able todeliver higher levels of output power with high efficiency compared toknown prior art apparatus.

One aspect of the present disclosure is a power oscillator apparatuscomprising: a first power oscillator and a second power oscillatorarranged in series between a supply voltage and a reference voltage, anelectromagnetic network for coupling the first and the secondoscillator, characterized by comprising a transformer with a primarycircuit including a first portion connected to the first oscillator andsecond portion connected to the second oscillator, a circuit element forreusing the current used in the first oscillator even into the secondoscillator, an output stage of the apparatus comprising a secondarycircuit of the transformer, the first and the second oscillator beingconfigured to receive a synchronizing signal for their synchronizationin frequency and phase and said secondary circuit being magneticallycoupled with the first and the second portion of the primary circuit toobtain an output power as combination of a first power associated to thefirst portion and a second power associated to the second portion of theprimary circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, a preferredembodiments thereof are now described, purely by way of non-limitingexample and with reference to the annexed drawings, wherein:

FIG. 1 shows a power oscillator apparatus according to the presentdisclosure;

FIG. 2 shows a power oscillator apparatus according to a firstembodiment of the present disclosure;

FIG. 3 shows a a power oscillator apparatus according to a secondembodiment of the present disclosure;

FIG. 4 shows a power oscillator apparatus according to a thirdembodiment of the present disclosure;

FIG. 5 shows the time diagrams of the some voltages of the poweroscillator apparatus in FIG. 2;

FIG. 6 shows more in detail the synchronizing circuit in FIGS. 2-4;

FIG. 7 is a schematic tridimensional view of an implementation of thestructure of the transformer of FIG. 2;

FIG. 8 is schematic planar view of the implementation of the structureof the transformer of FIG. 7;

FIG. 9 is schematic planar view of another implementation of thestructure of the transformer of FIGS. 2 and 3;

FIG. 10 is schematic planar view of an implementation of the structureof the transformer of FIG. 4; and

FIG. 11 is schematic planar view of another implementation of thestructure of the transformer of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a power oscillator apparatus according to the presentdisclosure.

The power oscillator apparatus comprises a first power oscillator POSCand a second power oscillator NOSC arranged in series between a supplyvoltage VDD and a reference voltage, for example ground GND.

The power oscillator apparatus comprises an electromagnetic network 100configured to couple the oscillators POSC and NOSC each one having twooutput terminals OUT1, OUT2.

The power oscillator apparatus comprises a transformer 50. The primarycircuit 51 comprises a first portion 52 connected to the firstoscillator POSC and a second portion 53 connected to the secondoscillator NOSC; the first portion 52 is connected with the outputterminals OUT1, OUT2 of the first oscillator POSC while the secondportion 53 is connected with the output terminals OUT1, OUT2 of thesecond oscillator NOSC.

The primary circuit of the transformer comprises preferably four primarywinding inductors L_(P1)-L_(P4) wherein the first portion 52 comprisestwo winding inductors and the second portion 53 comprises the other twowinding inductors.

The power oscillator apparatus comprises a circuit element 101 to allowthe reuse of the current I passing through the first oscillator eveninto the second oscillator NOSC; the circuit element 101 is preferablythe common center tap of the first 52 and second 53 portion of theprimary circuit 51 of the transformer 50.

The power oscillator apparatus receives a synchronizing signal Ipulsefor the synchronization in frequency and phase of the first POSC and thesecond NOSC oscillators; the synchronizing signal Ipulse derives from asynchronizing circuit 60, preferably included in the power oscillatorapparatus. The synchronization frequency f_(sync) of the thesynchronizing signal Ipulse is equal about to 2*f_(osc) where f_(osc) isthe oscillation on quency of the each oscillator NOSC, POSC. Thesynchronizing circuitry 60 forces the oscillators POSC and NOSC tooperate in phase, so that the voltages applied across the winding orcoils L_(P1)-L_(P4), denoted with the same symbol (i.e., dot or cross),are at the same time all positive or all negative.

The power oscillator apparatus comprises an output stage 70 includingthe secondary circuit L_(S1) and L_(S2) of the transformer which isconnectable with a load LOAD, for example a rectifier. The secondarycircuit L_(S1), L_(S2) is magnetically coupled with the primary circuitto obtain an output power Pout which is a power combining of a firstpower P1 associated to the first portion 52 of the primary circuit and asecond power P2 associated to the second portion 53 of the primarycircuit. The transformer 50 allows the galvanic isolation between theoscillators NOSC, POSC and the output stage 70 of the power oscillatorapparatus.

FIG. 2 shows a power oscillator apparatus according to a firstembodiment of the present disclosure. The oscillators POSC and NOSC areimplemented by complementary oscillators; the oscillators POSC and NOSCmay be implemented in either bipolar or CMOS technologies. FIG. 2 showsthe oscillators POSC and NOSC implemented in CMOS technologies.

The oscillator POSC comprises a first PMOS transistor M1 and a secondPMOS transistor M2 which have the source terminals connected to thesupply voltage VDD and are cross-coupled, that is the gate terminal ofthe transistor M1 is in common with the drain terminal of the transistorM2 and the gate terminal of the transistor M2 is in common with thedrain terminal of the transistor M1.

The oscillator NOSC comprises a first NMOS transistor M3 and a secondNMOS transistor M4 which have the source terminals connected to groundGND and the gate terminals connected by means of the resistances R3 andR4 with the bias voltage V_(B) at the bias terminal Pbias. Theoscillator NOSC comprises a capacitor C3 connected with the gateterminal of the transistor M3 and the drain terminal of the transistorM4 and another capacitor C4 connected with the gate terminal of thetransistor M4 and the drain terminal of the transistor M3.

The electromagnetic network 100 configured to couple the oscillatorsPOSC and NOSC is of the inductive type and comprises the primary circuit51 of the transformer 50. The primary circuit 51 comprises the firstportion 52 including the series of coils L_(P4) and L_(P3) associated tothe oscillator POSC and the second portion 53 including the series ofthe coils L_(P1) and L_(P2) associated to the oscillator NOSC; thecoupling between the oscillators POSC and NOSC is assured by themagnetic coupling of the coils L_(P4) and L_(P2) denoted by the symbolcross and the magnetic coupling of the coils L_(P1) and L_(P3) denotedby the symbol dot.

A capacitor C1 is connected between the drain terminals of thetransistors M1 and M2 and forms with the coils L_(P4) and L_(P3) aresonant tank LC while a capacitor C2 is connected between the drainterminals of the transistors M3 and M4 and forms with the coils L_(P1)and L_(P2) another resonant tank LC.

The secondary circuit of the transformer 50 comprises the series of thecoils L_(S1) and L_(S2) wherein the coil L_(S1) is magnetically coupledwith the coils L_(P1) and L_(P3) of the primary circuit and the coilL_(S2) is magnetically coupled with the coils L_(P2) and L_(P4) of theprimary circuit. The output power Pout relative to the series of thecoils L_(S1) and L_(S2) is a power combining of each power contributionP_(LP1)-P_(LP4) of the respective coil L_(P1), L₂, L_(P3) and L_(P4) ofthe primary circuit 51.

When the transistor M1 is on and the transistor M2 is off the current Iflows through the coils L_(P4) and L_(P2) and the transistor M4 whilewhen the transistor M2 is on and the transistor M1 is off the current Iflows through the coils L_(P3) and L_(P1) and the transistor M3. Thevalues of inductors L_(P1), L_(P2), L_(P3) L_(P4) and capacitors C1 andC2 are related to the oscillation frequency f_(OSC) that is typically inthe range between hundreds of megaHertz to several gigahertz. Therefore,in a typical integrated implementation of the proposed solutioninductors and capacitors of a few nanoHerny and picoFarad are used,respectively.

The synchronizing circuit 60 uses common-mode current pulses Ipulse. Thecurrent pulses are injected into the power oscillator apparatus by usinga common-mode bias terminal Pbias, which can be placed in either theoscillators NOSC or POSC and which, in FIGS. 2-4, is arranged in theoscillator NOSC; preferably a NMOS transistor M6 coupled between thecircuit 60 (coupled with the supply voltage VDD) and ground GND and withthe gate and drain terminal in common and with the drain terminalconnected with the circuit 60, allows the use of the terminal Pbias forthe injection of the current pulses Ipulse. Current pulses Ipulse have afrequency f_(sync) approximately equal to two times the oscillationfrequency f_(osc) of the oscillator POSC, NOSC; current pulses Ipulsehave preferably a square-wave shape. Preferably the synchronizingcircuitry 60, as shown in FIG. 6, includes a low-power low-accuracyvoltage oscillator 61 (e.g., a ring oscillator), a voltage-to-currentconverter 62 receiving the voltage pulses output from the oscillator 61,and a current buffer 62 receiving the current pulses Ipulse, for exampleof 1 mA, from the converter 62 and adapted to inject the current pulsesIpulse into the bias terminal Pbias.

The presence of the synchronizing signal Ipulse of the synchronizingcircuit 60 is mandatory to avoid NOSC and POSC work at differentfrequency/phase, thus hindering the power-combining at the output stage70. The synchronizing circuit 60 drives the second-harmonic(common-mode) current component to both NOSC and POSC, thus settingfrequency/phase of NOSC and POSC.

The synchronization signal has no impact on the oscillator efficiencysince low-value current pulses are required and synchronization is onlyrequired at the circuit start-up. Indeed, after the oscillator is lockedin a stable state, it remains indefinitely in this state, regardlesssignal disturbance.

FIG. 5 shows a the typical waveforms of the voltages at the coils of thepower oscillator apparatus in FIG. 2; Vout_NOSC is the differentialvoltage across the oscillator NOSC, i.e. the voltage across the seriescombination of L_(P1) and L_(P2) and Vout_POSC is the differentialvoltage across the oscillator POSC, i.e. the voltage across the seriescombination of L_(P3) and L_(P4) while Vout is the differential voltageacross the equivalent load LOAD, i.e. the voltage across the seriescombination of the secondary coils L_(S2) and L_(S1) which is greaterthan the voltages Vout_POSC and Vout_NOSC but smaller than their sum.

It is clearly shown that due to the phase-relationship between Vout_NOSCand Vout_POSC, the currents forced at the primary coils are at the sametime all increasing or all decreasing, and hence the fluxes generated atthe primary coils. It follows that the secondary coils will catch thisflux (separately, i.e. L_(S1) will catch the flux generated by L_(P1)and L_(P3) and so on), forcing to the load a current proportional to thefluxes. At the secondary side the output voltage will be greater thanVout_NOSC or Vout_POSC, depending on the load resistance and thecoupling factor between primary and secondary side, always less thanone. The total power at the load LOAD is the sum of the total powerapplied at the primary side, except for the losses in the seriesresistance of the windings.

FIG. 3 shows a power oscillator apparatus according to a secondembodiment of the present disclosure. Differently from the poweroscillator apparatus in FIG. 2, the electromagnetic network 100configured to couple the oscillators POSC and NOSC of the poweroscillator apparatus in FIG. 3 is of the capacitive type; in fact theelectromagnetic network 100 comprises the capacitor C1 connected betweenthe first output terminal OUT1 of the first oscillator and the secondoutput terminal OUT2 of the second oscillator and a second capacitor C2connected between the second output terminal OUT2 of the firstoscillator and the first output terminal OUT1 of the second oscillator,that is the capacitor C1 is connected between the drain terminal of thePMOS transistor M1 and the drain terminal of the NMOS transistor M4 andthe capacitor C2 is connected between the drain terminal of the PMOStransistor M2 and the drain terminal of the NMOS transistor M3.

Differently from the power oscillator apparatus in FIG. 2, the firstportion 52 of the primary circuit 51 of the transformer 50 comprises theseries of the coils L_(P2) and L_(P4) connected between the drainterminals of the PMOS transistors M1 and M2 and the second portion 53 ofthe primary circuit 51 comprises the series of the coils L_(P1) andL_(P3) connected between the drain terminals of the NMOS transistors M3and M4.

The secondary circuit of the transformer 50 comprises the series of thecoils L_(S1) and L_(S2) wherein the coil L_(S1) is magnetically coupledwith the coils L_(P1) and L_(P3) of the primary circuit and the coilL_(S2) is magnetically coupled with the coils L_(P2) and L_(P4) of theprimary circuit. The output power Pout relative to the series of thecoils L_(S1) and L_(S2) is a power combining of each power contributionP_(LP1)-P_(LP4) of the respective coil L_(P1), L_(P2), L_(P3) and L_(P4)of the primary circuit 51.

FIG. 4 shows a power oscillator apparatus according to a thirdembodiment of the present disclosure. Differently from the poweroscillator apparatus in FIG. 2, the electromagnetic network 100configured to couple the oscillators POSC and NOSC of the poweroscillator apparatus in FIG. 3 is of the capacitive and inductive type;in fact the electromagnetic network 100 comprises the capacitor C1connected between the first output terminal OUT1 of the first oscillatorPOSC and the second output terminal OUT2 of the second oscillator NOSCand a second capacitor C2 connected between the second output terminalOUT2 of the first oscillator and the first output terminal OUT1 of thesecond oscillator, that is the capacitor C1 is connected between thedrain terminal of the PMOS transistor M1 and the drain terminal of theNMOS transistor M4 and the capacitor C2 is connected between the drainterminal of the PMOS transistor M2 and the drain terminal of the NMOStransistor M3.

Also the electromagnetic network 100 comprises the primary circuit 51 ofthe transformer 50. The electromagnetic network 100 comprises the seriesof coils L_(P4) and L_(P3) associated to the oscillator POSC, that isconnected to the output terminals OUT1 and OUT2 of the oscillator POSC,and the series of the coils L_(P1) and L_(P2) associated to theoscillator NOSC, that is connected to the output terminals OUT1 and OUT2of the oscillator NOSC, the coupling between the oscillators POSC andNOSC is assured by the magnetic coupling of the coils L_(P4) and L_(P2)denoted by the symbol cross and the magnetic coupling of the coilsL_(P1) and L_(P3) denoted by the symbol dot.

For all the embodiments in FIGS. 2-4, the transformer topology comprisestwo separated magnetic circuits, whose common fluxes are marked by dots(i.e., L_(P1,3) with L_(S1)) and crosses (i.e., L_(P2,4) with L_(S2)),respectively. Dots and crosses are placed according to the common fluxconventions.

In accordance with the power oscillator apparatus of the presentdisclosure it is possible to perform an integrated circuit comprisingthe power oscillator apparatus as shown in each one of the FIGS. 1-4.The integrated circuit shows a physical monolithic implementation forthe transformer 50 using only three metal layers. FIG. 7 shows aschematic tridimensional view of the structure of the transformer 50while FIGS. 8-11 show schematic planar views of primary L_(P1)-L_(P4)and secondary L_(S1), L_(S2) windings. FIG. 8 is the planar view oftransformer 50 related to the tridimensional view of FIG. 7. A stackedarrangement for the transformer 50 comprises the primary coilsL_(P1)-L_(P4) performed in the mid-level or intermediate metal layer 55and the secondary coils L_(S1), L_(S2) performed in the top metal layer56; preferably the primary coils L_(P1)-L_(P4) and the secondary coilsL_(S1), L_(S2) are provided in the form or metal spirals. The commoncenter tap 101 may be performed in the bottom metal layer 57 or in theintermediate metal layer 55. The integrated circuit is performed in achip of semiconductor material and the transistors M1 -M4 and the otherelements of the oscillators POSC and NOSC except the transformer 50 areperformed according to the known technology.

The four inductors L_(P1)-L_(P4) of the primary coils are arranged usingtwo symmetric interleaved configurations, one for each secondary coupledcoils L_(S1), L_(S2), with a common terminal for the center-tap 101.Underpasses are performed in the bottom metal layer 57 and are only usedto contact the inductors terminals and preferably the center-tap 101.Secondary coils L_(S1), L_(S2) are stacked on top of primary coilsL_(P1)-L_(P4) and series-connected to build the secondary winding. Theirinner terminals are contacted by bonding wires. The primary coilsL_(P1), L_(P3) (with the winding L_(P1) in black and the winding L_(P3)in white) are arranged in a interleaved configuration under thesecondary coil L_(S1) and the primary coils L_(P2), L_(P4) (with thewinding L_(P2) in black and the winding L_(P4) in gray) are arranged ina interleaved configuration under the secondary coil L_(S2).

The stacked configuration between primary and secondary windings isinherently suitable to obtain galvanic isolation, provided that suitabledielectric layer between the intermediate metal layer 55 and the topmetal layer 56 is used. For the sake of clarity, FIGS. 7-11 are only anexample of implementation. Indeed, the shape, the number of turns andthe turn ratio between primary and secondary windings may vary.Moreover, if more metal layers are available, multi-layershunt-connected spirals can be exploited to reduce the seriesresistances of the coils. Patterned ground shields can be implementedbelow the primary windings to reduce substrate losses if necessary.

For both schematics in FIGS. 2 and 3, an alternative implementation ofthe transformer is reported in the planar view in FIG. 9. It mainlydiffers from the one shown in FIG. 8 for the magnetic fields B that arein opposite phase between coils at the left side, L_(P1), L_(P3) andL_(S1), and the right side, L_(P2), L_(P4) and L_(S2), of the structure.This configuration allows lower electromagnetic interferences to beachieved.

For the schematic in FIG. 4 two alternative implementations of thetransformer 50 are shown in FIGS. 10 and 11. These implementations usetwo different interleaved transformers at the primary side, while thesecondary is the same as the previous solutions in FIGS. 8 and 9,respectively. FIG. 10 shows the primary coils L_(P1), L_(P3) arranged inan interleaved configuration under the secondary coil L_(S1) and theprimary coils L_(P2), L_(P4) arranged in an interleaved configurationunder the secondary coil L_(S2) with the magnetic fields B that are inphase between the coils while FIG. 11 shows the primary coils L_(P1),L_(P3) arranged in an interleaved configuration under the secondary coilL_(S1) and the primary coils L_(P2), L_(P4) (with the winding L_(P2) inwhite and the winding L_(P4) in black) arranged in an interleavedconfiguration under the secondary coil L_(S2) with the magnetic fields Bthat are in opposite phase between coils at the left side, L_(P1),L_(P3) and L_(S1), and the right side, L_(P2), L_(P4) and L_(S2), of thestructure.

Compared to the implementations in FIGS. 8 and 9 this arrangement needsonly one underpass.

Compared to the typical apparatuses, the power oscillator apparatusshown in FIG. 1-11 is able to deliver higher levels of power, whileproviding higher efficiency. Indeed, it is able to overcome thelimitations of the oscillating voltage due to the breakdown voltagethanks to a transformer-based power combining technique. The efficiencyis further increased thanks to the current-reuse approach. Finally, themixed stacked-interleaved configuration that is proposed for thetransformer implementation allows low-area consumption to be achieved.The transformer structure is inherently suited for (integrated) galvanicisolation, provided that a proper dielectric layer is used. Moreover, itis easy to obtain a high voltage boosting ratio between the secondaryand the primary side by taking advantage of the number of turn ratio.

It is worth noting that when inductive coupling is adopted betweenprimary coils, as in the configurations shown in FIGS. 2, 3 and 4, theequivalent resonator inductance, L_(eq) is increased according to thefollowing expression:

L _(eq)=(L _(P1,3) +L _(P2,4))·(1+kp)

where k_(P) is the magnetic coupling factor between the primary coils.This achievement allows obtaining a significant area reduction comparedto no-coupled coils. The proposed invention can be implemented indifferent approaches: in a monolithic solution, using a post-processingfor the dielectric and the secondary coil, using two face-to-face dicewith a post-processing for the dielectric, as the approach described inUnited State Patent Application Publication No. 2012/0256290(incorporated herein by reference) or using a system-in-package approachwith a post-processed transformer according to the Analog Device Inc.isoPower® technology.

A non-limiting design implementation of the apparatus shown in FIG. 2 isreported below for a typical 0.35-μm CMOS process. Consideringf_(osc)=250 MHz, f_(sync)=500 MHz, Ipulse=1.5 mA, VDD=3 V,L_(P1)=L_(P2)=L_(P3)=L_(P4)=5 nH, L_(S1)=L_(S2)=10 nH,k_(P1,3)=k_(P2,4)=k_(P)=0.6 (i.e., magnetic coupling factor betweenprimary coils 51), k_(P1,3-S1)=k_(P2,4-S2)=0.8 (i.e., magnetic couplingfactor between primary coils 51 and secondary coils in the output stage70), C1=C2=17 pF (excluding the parasitic capacitor of active devicesM1-M4), C3=C4=10 pF, LOAD=60Ω, R3=R4=1 kΩ. The circuit behavior can beexplained as the superposition of two in-phase oscillators (i.e., NOSCand POSC) in which, as in classical cross-coupled topologies,transistors M1-M2 and M3-M4 provide the non-linear negative conductancerequired to sustain the steady-state oscillation. The cross-coupledconnection in the NOSC is guaranteed by the high-pass RC circuit formedby R3-C3 and R4-C4, respectively, thus allowing a bias terminal Pbias tobe available for the connection of the biasing/synchronizationcircuitry. The oscillator resonant tanks are the LC parallel networksmade up by L_(P1), L_(P2), C1 and L_(P3), L_(P4), C2 for the NOSC andPOSC, respectively. The tanks are tuned at about f_(osc) and thereforethe differential voltage waveforms at the output terminals (i.e., OUT1and OUT2) of each oscillator are forced to be almost sinusoidal atf_(osc). The presence of magnetic couplings between primary coils 51,increases the equivalent inductance according to the followingexpression: L_(eq)=(L_(P1,3)+L_(P2,4))·(1+kp). The phase-relationshipbetween Vout_NOSC (i.e., the voltage between the terminals OUT1-OUT2 ofthe oscillator NOSC) and Vout_POSC (the voltage between the terminalsOUT1-OUT2 of the oscillator POSC) is due to the primary couplingconfiguration, as well as the common-mode synchronizing signal atf_(sync) (i.e., at 2 times f_(osc)). Therefore, the currents forced atthe primary coils are at the same time all increasing or all decreasing,and hence the fluxes generated at the primary coils. It follows that thesecondary coils will catch this flux (separately, i.e. L_(S1) will catchthe flux generated by L_(P1) and L_(P3) and so on), forcing to the loada current proportional to the fluxes. At the secondary side the outputvoltage will be greater than Vout_NOSC or Vout_POSC. The total power atthe load is the sum of the total power applied at the primary side,except for the losses in the series resistance of the windings.

To deliver high level of power with high efficiency, transistors M1-M4have to work as switches with very low on resistances. Moreover, theloss reduction in the transformer is highly related to the availabilityof low-resistance metal layers (55, 56 and 57) to obtain highquality-factor coils.

What is claimed is:
 1. An apparatus, comprising: a first poweroscillator and a second power oscillator arranged in series between asupply voltage and a reference voltage, an electromagnetic networkconfigured to couple the first and the second oscillator, saidelectromagnetic network comprising a transformer with a primary circuitincluding a first portion connected to the first oscillator and secondportion connected to the second oscillator, a circuit element configuredto reuse current used in the first oscillator even into the secondoscillator, an output stage comprising a secondary circuit of thetransformer, wherein the first and the second oscillator are configuredto receive a synchronizing signal for synchronization of frequency andphase, and wherein said secondary circuit is magnetically coupled withthe first and the second portion of the primary circuit to obtain anoutput power as combination of a first power associated to the firstportion and a second power associated to the second portion of theprimary circuit.
 2. The apparatus according to claim 1, wherein thefirst portion and the second portion of the primary circuit eachcomprise at least two windings, said circuit element configured to reusethe current comprising a common center tap of the first portion and thesecond portion of the primary circuit.
 3. The apparatus according toclaim 1, wherein said electromagnetic network configured to couple thefirst and second power oscillators is of the capacitive type.
 4. Theapparatus according to claim 3, wherein each one of said first andsecond oscillators has a first output terminal and a second outputterminal, said electromagnetic network comprising a first capacitorcoupled between the first output terminal of the first oscillator andthe second output terminal of the second oscillator and a secondcapacitor coupled between the second output terminal of the firstoscillator and the first output terminal of the second oscillator. 5.The apparatus according to claim 1, wherein said electromagnetic networkcomprises an inductive coupling.
 6. The apparatus according to claim 1,wherein each of the first and second oscillators has a first outputterminal and a second output terminal, said electromagnetic networkcomprising the first portion and the second portion of the primarycircuit each respectively connected between the first and the secondoutput terminals of said first and second oscillators, each of the firstportion and the second portion of the primary circuit comprising atleast two windings, wherein one winding of the first portion ismagnetically coupled with one winding of the second portion.
 7. Theapparatus according to claim 6, wherein one winding of the first portionis magnetically cross-coupled with one winding of the second portion. 8.The apparatus according to claim 7, wherein said electromagnetic networkcomprises a capacitive coupling.
 9. The apparatus according to claim 8,wherein said electromagnetic network comprises a first capacitor coupledbetween the first output terminal of the first oscillator and the secondoutput terminal of the second oscillator and a second capacitor coupledbetween the second output terminal of the first oscillator and the firstoutput terminal of the second oscillator.
 10. The apparatus according toclaim 1, wherein the synchronizing signal has a synchronizationfrequency equal to two times the oscillation frequency of the each ofthe first and second oscillators.
 11. The apparatus according to claim2, wherein said secondary circuit comprises two windings, each windingof the secondary circuit being magnetically coupled with two windings ofthe primary circuit.
 12. The apparatus according to claim 10, furthercomprising a common-mode bias terminal provided in one of said first andsecond oscillators for receiving said synchronizing signal.
 13. Theapparatus according to claim 10, further comprising a synchronizingcircuit configured to generate said synchronizing signal, saidsynchronizing signal comprising common-mode current pulses.
 14. Theapparatus of claim 1, fabricated as an integrated circuit wherein thetransformer is formed in a stacked arrangement, wherein the secondarycircuit is formed in a top metal layer, wherein the primary circuit ofthe transformer is formed in an intermediate metal layer and wherein thecircuit element is formed in one of a bottom metal layer or theintermediate metal layer.
 15. The apparatus according to claim 14,wherein the primary circuit comprises a first pair and a second pair ofwindings and the secondary circuit comprises a pair of windings, eachpair of windings of the primary circuit being formed in interleavedconfiguration under one respective winding of the pair of windings ofthe secondary circuit.
 16. An apparatus, comprising: a first oscillatorcircuit having a first output and a second output; a second oscillatorcircuit having a first output and second output; a transformer having aprimary winding coupling the first and second outputs of the firstoscillator circuit to the first and second outputs of the secondoscillator circuit and further including a secondary windingmagnetically coupled to the primary winding; wherein the first andsecond oscillator circuits and the transformer are integrated in anintegrated circuit device including a plurality of metallization levels;and wherein said primary and secondary windings are formed in saidplurality of metallization layers.
 17. The apparatus of claim 16,wherein said primary winding comprises: a first winding and a secondarywinding coupled in series at a first tap and coupled between the firstoutput and second output of the first oscillator circuit; a thirdwinding and a fourth winding coupled in series at a second tap andcoupled between the first output and second output of the secondoscillator circuit; and a circuit element connecting the first tap tothe second tap.
 18. The apparatus of claim 17, wherein said firstthrough fourth windings are formed in a first layer of saidmetallization layers and said secondary winding is formed in a secondlayer of said metallization layers.
 19. The apparatus of claim 18,wherein said first and third windings are interleaved and wherein saidsecond and fourth windings are interleaved.
 20. The apparatus of claim19, wherein said secondary winding comprises a fifth winding and a sixthwinding coupled in series, and wherein said fifth winding is formed insaid second layer over the interleaved first and third windings formedin said first layer and wherein said sixth winding is formed in saidsecond layer over the interleaved second and fourth windings formed insaid first layer.
 21. The apparatus of claim 17, wherein said firstoscillator includes a first capacitor coupled between the first outputand second output of the first oscillator circuit and wherein saidsecond oscillator includes a second capacitor coupled between the firstoutput and second output of the second oscillator circuit.
 22. Theapparatus of claim 17, wherein said first oscillator includes a firstcapacitor coupled between the first output of the first oscillatorcircuit and the second output of the second oscillator circuit andwherein said second oscillator includes a second capacitor coupledbetween the first output of the second oscillator circuit and the secondoutput of the first oscillator circuit.
 23. The apparatus of claim 16,further comprising a synchronization circuit configured to generate asynchronizing signal for application to at least one of the first andthe second oscillators for synchronization of frequency and phase.